Sabkha and Burrow-Mediated Dolomitization in theMississippian Debolt Formation, NorthwesternAlberta, Canada

Greg M. Baniak, Larry Amskold, Kurt O. Konhauser, Karlis Muehlenbachs, S. George Pemberton,and Murray K. Gingras

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada

Dolomitized burrows in the Mississippian (Visean) DeboltFormation of northwestern Alberta, Canada form the primaryreservoir intervals in the Dunvegan gas field. Sedimentologicaland ichnological analyses suggest a carbonate ramp setting thatincludes subenvironments such as sabkhas, hypersaline lagoons,restricted subtidal lagoons, intertidal mud flats, and peloidalshoals. Dolomitization occurs primarily within oxidized muds andhighly bioturbated sediments, with the primary mode beingsabkha-associated precipitation. In this context, dolomitizationwithin the burrows also appears to be mediated by sulfate-reducing bacteria. d18O values for dolomite within burrows (mean2.4%) are enriched by 1.3% relative to calcite values (mean 1.1%)within the burrows. This degree of fractionation is similar fordolomite and calcite that have precipitated from the same solution.It is therefore suggested that the protodolomite precipitated inequilibrium with calcite rather than by replacement of pre-existing calcite. Isotopic values of d13C measured for dolomiteassociated with burrows (mean 3.4%) and matrix (mean 3.5%) isslightly enriched relative to measured calcite values (mean 3.2%for matrix; mean 3.1% for burrows). These isotopic trends arecommon for modern dolomite that has precipitated in equilibriumwith seawater where concomitant sulfate reduction and organiccarbon-oxidation is inferred to occur near the surface.

Keywords Ichnology, Facies, Isotopes, Burrow-associated dolo-mite, Sabkha, Bacterial sulfate reduction

INTRODUCTION

The study of dolomite has a long and extensive history,

dating back to the pioneering work of Giovanni Arduino and

D�eodat Gratet de Dolomieu in the 18th century. Despite being

an abundant mineral in the rock record, dolomite distribution

is limited in modern sedimentary environments. Kinetic

energy barriers such as high hydration energies of the magne-

sium ion (Mg2C) (Markham et al., 2002), low concentration

and activities of the carbonate ion (CO32¡), and the presence

of sulfate ions (Morrow, 1982; Budd, 1997) are all major

inhibiting factors in abiotic dolomite precipitation. Despite

this knowledge, replicating dolomite precipitation under mod-

ern physico-chemical conditions in the laboratory has proven

futile (McKenzie, 1991; Land, 1998). The formation of dolo-

mite and the nature of the dolomitizing fluids remains contro-

versial and is widely referred to as the “dolomite problem”

(Gunatilaka, 1987; Arvidson and Mackenzie, 1999; Machel,

2004). Classic models such as evaporative pumping, hydro-

thermal circulation, seepage reflux, sabkha, seawater convec-

tion, burial diagenesis, and subsurface mixing have been

invoked to explain dolomitization within the rock record (Hs€uand Siegenthaler, 1969; Morrow, 1982; Machel and Mountjoy,

1986; Warren, 2000; Machel, 2004).

In this context, burrow-associated diagenesis has recently

emerged as another model for shallow-burial dolomitization.

This is due to textural and compositional heterogeneities (e.g.,

concentration of extracellular polysaccharides in burrow walls

and contrasts in permeability) associated with biogenic sedi-

mentary structures (Gingras et al., 2004; Rameil, 2008; Corlett

and Jones, 2012). Although occurrences of dolomitized bur-

rows within the rock record are common (e.g., Baniak et al.,

2013; Beales, 1953; Kendall 1977a; Morrow, 1978; Zenger,

1996; Keswani, 1999), the timing and cause of dolomitization

associated with the burrows can be difficult to determine.

Within modern environments, microbial sulfate reduction

(MSR) plays an integral role in the precipitation of dolomite

at near-surface temperatures (Vasconcelos et al., 1995;

Address correspondence to Greg M. Baniak, Department of Earthand Atmospheric Sciences, 1-26 Earth Sciences Building, Universityof Alberta, Edmonton, Alberta, Canada, T6G 2E3. E-mail:baniak@ualberta.ca.

Greg M. Baniak’s current affiliation is BP Canada Energy GroupULC, Calgary, Alberta, Canada. Larry Amskold’s current affiliationis Jacobs Engineering, 4300 B Street, Suite 600 Anchorage, AK99503, USA.

Color versions of one or more of the figures in the article can befound online at www.tandfonline.com/gich.

158

Ichnos, 21:158–174, 2014

Copyright � Taylor & Francis Group, LLC

ISSN: 1042-0940 print / 1563-5236 online

DOI: 10.1080/10420940.2014.930036

Warthmann et al., 2000; van Lith et al., 2003; Krause et al.,

2012). Examples include the hypersaline lagoons of Lagoa

Vermelha and Brejo do Espinho, northeastern coast of Brazil

(Vasconcelos et al., 1995, 2005; Vasconcelos and Macken-

zie, 1997; Warthmann et al., 2005) and the ephemeral lakes

of the Coorong Region of South Australia (Von der Borch

and Lock, 1979; Wright and Wacey, 2005; Wacey et al.,

2007). Such studies are invaluable as they help outline the

manner in which microbes influence the physiochemical

parameters that can help mediate early dolomitization

within modern sediments.

The focus of this study is the Mississippian (Visean) Debolt

Formation located in the Dunvegan gas field of northwestern

Alberta (Fig. 1). Located at depths greater than 1400 m, the

Dunvegan gas field has reserves within a 3.4 £ 1010 m3 to 4.5

£ 1010 m3 (1.2 to 1.6 trillion cubic feet) range (Al-Aasm and

Packard, 2000; Packard et al., 2004). These reserves make the

Dunvegan gas field the second largest carbonate-hosted non-

reefal gas field in the Western Canadian Sedimentary Basin

(Al-Aasm and Packard, 2000). Reservoir production occurs

primarily from highly porous microsucrosic dolomites occur-

ring within the bioturbated fabrics (Packard et al., 2004). The

preferred dolomitization of the burrow fabrics suggests certain

parameters (e.g., the presence of MSR, low oxygenation, high

salinity) must have existed. The purpose of this article is to

document the lithofacies and their paleogeographic distribu-

tion within the Dunvegan gas field. From this, an understand-

ing of the environmental settings that helped mediate

dolomitization within the burrows will be analyzed by inte-

grating lithologic, petrographic, and isotopic analysis.

FIG. 1. Location map of the Dunvegan gas field in northwestern Alberta that produces from the Mississippian (Visean) Debolt Formation. In the study area, 15

cored wells (demarcated in blue) were studied. The wells used in the cross-section (A-A’, Fig. 4) are demarcated in red. (See Color Plate I.)

BURROW-MEDIATED DOLOMITIZATION DEBOLT FORMATION 159

IMPACT OF BIOTURBATION ON SUBSTRATEBIOGEOCHEMISTRY

Infaunal invertebrates, such as crustaceans, insects, and

earthworms, commonly modify sediment through particle

remobilization and grazing (Aller, 1994; Bromley, 1996;

Gingras et al., 2007a). A common by-product of bioturbation

is the introduction of labile organic matter, such as mucous

secretions and fecal material, into the substrate (Steward et al.,

1996; Hauck et al., 2008; Pak et al., 2010). Mucous secretions,

for instance, represent ideal microenvironments for microbes

since many of the burrows are lined with organic materials

such as extracellular polysaccharides (i.e., microbial exopoly-

mer secretions) and glycoprotein mucin (Konhauser and

Gingras, 2007, 2011; Lalonde et al., 2010; Petrash et al.,

2011). For example, Gunnarsson et al. (1999a) observed that

elevated concentrations of a tetrachlorobiphenyl commonly

occurred in the thin mucous layer covering the burrow linings

of the polychaete Neries diversicolor compared to the sur-

rounding bulk sediment. Similar observations were also made

by Gunnarsson et al. (1999b) for the linings of disk chamber

and arm burrows of the brittle star Amphiura filiformis. These

microbial exopolymer secretions (EPS) are important, as they

are able to bind organic compounds of both high and low

molecular weight (Decho, 1990). As a result, EPS comprises

a significant portion of organic carbon within sediment

(Confer and Logan, 1998) and acts as a reactive interface for

the sorption of dissolved matter and secondary mineral precip-

itation (Mayer et al., 1999; Gunnarsson et al., 1999a, 1999b;

Konhauser and Gingras, 2007, 2011).

The eventual decomposition of EPS and other organic

material within bioturbated environments is critical in driving

sedimentary metabolic reactions. In short, any organic carbon

present usually becomes paired to a sequence of terminal elec-

tron accepting processes, with the more energetically favored

reactions proceeding first (Froelich et al., 1979; Canfield et al.,

1993; Chapelle et al., 1995). As a result of the decomposition

of EPS, idealized biogeochemical reactions within sediments

(e.g., Freolich et al., 1979) can become disrupted through

activities such as bioturbation (Aller, 1982; Kristensen, 2000;

Needham et al., 2004). In any event, these geochemical reac-

tions are commonly microbially mediated and include (after

Froelich et al., 1979 and Konhauser, 2007):

[1] Aerobic respiration:CH2O (organic carbon)C O2 ! CO2 C H2O

[2] Nitrate reduction:2CH2O C NO3

¡ C 2HC ! 2CO2 C NH4C C H2O

[3] Mn(IV)-oxide reductive dissolution:CH2O C 2MnO2(s) C 4HC ! CO2 C 2Mn2C C 3H2O

[4] Fe(III)-oxyhydroxide reductive dissolution:CH2O C 4FeOOH(s) C 8HC! CO2 C 4Fe2C C 7H2O

[5] Sulfate reduction:2CH2O C SO4

2¡ C HC ! HS¡ C 2CO2 C 2H2O

[6] Methanogenesis:2CH2O! CH4 C CO2

Notably, the oxidation (or mineralization) of organic carbon

leads to an increase in alkalinity (represented as CO2) and an

increase in pH (reactions 2 through 5). Studies of pore waters

in deep-sea sediments (e.g., Froelich et al., 1979) and anoxic

basins (e.g., Reeburgh, 1980) have shown that the most ener-

getically favorable redox reactions occur first and concentra-

tions of electron acceptors such as O2 and NO3 are typically

the first to become depleted. Sulfate reduction (reaction 5) and

methanogenesis (reaction 6) are therefore later-stage redox

reactions. Consequently, high concentrations of organic car-

bon are required to consume these earlier terminal electron

acceptors before the onset of the later reactions can occur.

Notably, the contemporaneous bacterial mineralization of

organic carbon and increase in pH will also help favor the for-

mation of a carbonate ion due to the hydration of CO2 to car-

bonic acid (H2CO3) and subsequent deprotonation reactions

(i.e., H2CO3 ! HCO3¡ C HC ! CO3

2¡ C 2HC).Modern studies by Garrison and Luternauer (1971) and

Gunatilaka et al. (1987) have shown that organic material

present within animal burrows helps promote the precipitation

of dolomite. Given the ubiquitous presence of organic material

within most burrows environments, development of a reducing

microenvironment can occur (Gingras et al., 2004). Within

reducing sediment, sulfate-reducing bacteria are present and

are able to remove dolomite-inhibiting sulfate anions. As a

result, liberation of the Mg2C cation from the SO42¡-Mg2C ion

pair occurs, thereby increasing the availability of the free

Mg2C cations for early dolomite precipitation (van Lith et al.,

2003). Elevated concentrations of sulfide have been measured

in modern burrows (Waslenchuk et al., 1983), suggesting that

sulfate reduction is indeed occurring within or adjacent to bur-

row systems.

REGIONAL GEOLOGICAL SETTING OF THEDEBOLT FORMATION

During the Mississippian and Pennsylvanian Periods in

western Canada, the principal tectonic features were (i) Prophet

Trough, (ii) Peace River Embayment, (iii) cratonic platform,

(iv) Williston Basin, and (v) Yukon Fold Belt (Richards,

1989). The Debolt Formation and its stratigraphic equivalents

are deposited primarily within the Prophet Trough and Peace

River Embayment in British Columbia and Alberta (Fig. 2).

The Prophet Trough is a downfaulted margin (i.e., distal fore-

land basin) that extends from the northern Yukon and Alaska

southward to join the Antler Foreland Basin of the western

United States (Richards, 1989; Richards et al., 1993, 1994).

The Peace River Embayment (Douglas et al., 1970) can trace

its origins to the structural evolution of the Peace River Arch

(Cant, 1988; O’Connell et al., 1990). The Peace River Arch

was a prominent east-northeast trending topographic high of

Precambrian granites that were approximately 800–1,000 m

above the regional elevation near the Alberta-British Columbia

border during the early Paleozoic (Cant, 1988).

160 G. M. BANIAK ET AL.

Despite providing expressible topographic relief from the

Cambrian to Devonian, a combination of subsidence (compres-

sion and extension) and normal faulting (horst and graben struc-

tures) to the Peace River Arch during the mid-Devonian to

Permian provided locations for sediments to accumulate (Cant,

1988; O’Connell et al., 1990). As a result of subsidence and nor-

mal faulting, the Peace River Arch became a negative feature

from theMississippian to Jurassic (Hein, 1999). Isopach maps of

northwestern Alberta show the Debolt Formation to be the thick-

est in areas interpreted to be bounded by extensional faults

(Cant, 1988). The Peace River Embayment may have become

connected to the Prophet Trough during the early- and middle-

Tournaisian due to the faulting and regional subsidence that con-

tinued throughout the Mississippian and Pennsylvanian (Dur-

ocher and Al-Aasm, 1997). Connection with the Prophet Trough

helped facilitate deposition of a thick succession within the

embayment and across the western Canadian Cratonic Platform

(O’Connell et al., 1990). As a result of these aforementioned

events, both stratigraphic (anhydrite, shale accumulations,

pinch-outs of dolomitized burrow intervals) and structural (two-

way up dip closures) petroleum traps formed within the Dunve-

gan gas field (Cioppa et al., 2003).

FIG. 2. A map of western Canada showing the depositional limits for sedimentation during the Mississippian and Pennsylvanian (Carboniferous Period). Non-

highlighted areas denote positive regions where sediments are not preserved. The location of interest for this study is situated in the southeastern margin of the

Peace River Embayment. The Debolt Formation corresponds to middle depositional unit (Rundle Assemblage) deposited during the Visean. Inset shows the

main physiographic provinces of western Canada. Figure modified from Richards et al. (1993, 1994).

BURROW-MEDIATED DOLOMITIZATION DEBOLT FORMATION 161

STRATIGRAPHIC SETTING

The Mississippian succession in the Peace River Embay-

ment of northwestern Alberta and interior platform of north-

eastern British Columbia is subdivided into three main

stratigraphic units. In chronological order, these units include

(i) lower to middle Tournaisian Banff Formation, (ii) middle

Tournaisian to upper Visean Rundle Group, and (iii) upper

Visean to Serpukhovian Stoddart Group (O’Connell, 1994)

(Fig. 3). The Debolt Formation is Visean in age and belongs

to the Rundle Group. Consisting primarily of carbonate and

evaporitic facies, the Debolt Formation typically grades into

deeper basinal facies towards the west (Richards, 1989;

Durocher and Al-Aasm, 1997; Al-Aasm and Packard, 2000).

Macauley (1958) was the first to subdivide the Debolt Forma-

tion into lower and upper members. Law (1981) later subdi-

vided the Debolt Formation into five informal members in

northeastern British Columbia. The Debolt Formation is

244 m thick at its type section in west-central Alberta and

becomes progressively thinner near its southwestern deposi-

tional limit (Richards et al., 1993). Within the Peace River

Embayment, the Debolt Formation overlies the Shunda

Formation in the east and the Pekisko Shale (Formation F) in

the west (Richards et al., 1993). The Golata Formation discon-

formably overlies the Debolt Formation throughout the

embayment. The outcrop nomenclature of the Mount Head

and Livingstone formations have been applied to internal

zones of the Debolt Formation with varying degrees of success

(Packard et al., 2004).

In general, the Debolt Formation records no less than 30

shoaling-upward hemicycles or fourth/fifth order parasequen-

ces (Al-Aasm and Packard, 2000; Packard et al., 2004). These

numerous packages are a reflection of the ongoing relative sea

level oscillation in a shallow-water to littoral setting during

the Visean. In Alberta, an early Visean transgression is

recorded by the presence of protected-shelf carbonates in the

basal part of the Debolt Formation above the restricted-shelf

carbonates of the Shunda Formation (Packard et al., 2004).

The subsequent regression, recorded by anhydrite and

restricted-shelf carbonates preserved in the upper Debolt For-

mation of west-central Alberta and east-central British Colum-

bia, appears to have coincided with sedimentation of the

regressive Wileman and Salter members of the Mount Head

Formation (Richards et al., 1993). Towards the east and east-

southeast into Alberta, the upper member of the Debolt Forma-

tion becomes anhydritic and displays sabkha cycles. Reservoir

zones in the Dunvegan gas field are located in the uppermost

argillaceous unit (Al-Aasm and Packard, 2000). This argilla-

ceous unit consists of a succession of stacked sabkha cycles of

thinly interstratified carbonates, evaporites, and siliclastics,

which are individually less than 2 m thick (Al-Aasm and Pack-

ard, 2000). A stratigraphic cross-section (Fig. 4) along a tran-

sect through the Dunvegan gas field illustrates the cyclical

nature of the sabkha cycles.

DATABASE ANDMETHODS

In this study, 15 drill cores from the Dunvegan gas field

were examined (see Fig. 1) along with 30 thin sections. The

thin sections from the drill core samples were examined under

a polarizing microscope. Sedimentological characterization

included identification of carbonate grains (e.g., ooids, pisoids,

oncoids, and other allochems), fossils, bedding contacts, layer

thicknesses, primary and secondary physical structures, litho-

logic constituents, and accessory minerals. Ichnological data

comprised descriptions of ichnogenera, cross-cutting relation-

ships, and bioturbation intensity.

Isotopic analyses (d18O and d13C) were performed on the

burrow fill and surrounding matrices. Samples for isotopic

analysis were obtained from representative sections of slabbed

core and were mechanically extracted using the engraver tip of

a high-speed rotary drill. The samples were selectively

obtained from locations where discrete burrows were present

or where burrows were absent (i.e., matrix). In each case,

10 to 25 mg samples were excavated from a depth of around

1 to 2 mm. In the case of burrow samples, this occasionally

FIG. 3. Subsurface stratigraphic column of the Mississippian Period for the

Peace River Embayment of northwestern Alberta and interior platform of

northeastern British Columbia (after Richards et al., 1993; Durocher and Al-

Aasm, 1997).

162 G. M. BANIAK ET AL.

necessitated sampling more than one burrow in an interval.

Samples for the isotopic analysis of the calcite phase were

obtained by reaction with 100% phosphoric acid (H3PO4) for

one hour at 25�C using the technique outlined by McCrea

(1950). Samples for the isotopic analysis of the dolomite phase

were obtained after an additional 24 hours in 100% H3PO4 at

50�C. Isotopic analyses were performed with a Finnigan-MAT

252 mass spectrometer operated in dual inlet mode. All isoto-

pic values are reported in standard delta notation (d) relative to

PDB standards (Craig, 1957).

RESULTS

Facies Analysis

In this study, the Dunvegan gas field can be subdivided into

six reoccurring facies intervals: nodular and bedded anhydrite

(Facies 1), microbially laminated mudstones (Facies 2), mas-

sive-patterned mudstone (Facies 3), bioturbated mudstone-

wackestone (Facies 4), skeletal packstone-wackestone (Facies

5), and peloidal grainstone-packstone (Facies 6). Within the

study area, Al-Aasm and Packard (2000) identified oxidized

muds and highly bioturbated sediments as the main intervals

with dolomite present. In this context, the bioturbated mud-

stones-wackestones (Facies 4) represents the primary focus for

this study regarding petrographic and isotopic analysis.

Facies 1 (F1): Nodular and Bedded Anhydrite

Description. Facies 1 consists of dense anhydrite and anhy-

dritic-dolomite that is nonbioturbated. The most commonly

observed type of anhydrite is nodular, with mottled and cryp-

tocrystalline fabrics and occasional dolostone stringers. Nod-

ules vary from 1 to 6 cm in diameter and are ovoid to irregular

in shape (Figs. 5A-B). Facies 1 is typically embedded within

dolostone and commonly has thicknesses ranging from 10 to

30 cm. Expressions of anhydrite in subvertically elongated

position, with crystals ranging up to 10 cm in length, are also

present (Fig. 5C). Examples of more massive nodular anhy-

drite, reaching up to 2 m in thickness, also occur within F1.

This unit contains larger nodules (up to 10 cm) and typically

contains little matrix. In areas, the nodules are present within,

or juxtaposed, to stylolites or pressure dissolution seams

FIG. 4. Stratigraphic cross-section of the Debolt Formation along the northwestern part of the Dunvegan gas field. All well-logs shown are gamma ray. Twelve

parasequence surfaces are recognized in this cross-section and are commonly represented in core as thinly bedded shale surfaces. Within these parasequences are

stacked sabkha facies that are gradational with tidal flat, lagoonal, and shoal sediments. The repetitive nature of these facies over a large stratigraphic interval

indicates high levels of sea level oscillation (i.e., transgressive-regressive cycles) in a shallow-water ramp setting during the Visean. (See Color Plate II.)

BURROW-MEDIATED DOLOMITIZATION DEBOLT FORMATION 163

(Fig. 5D). Examples of bedded anhydrite containing thin inter-

beds of dolostone are also present in localized areas. These

bedded anhydrites range up to 10 m in thickness and preserve

planar to wavy laminations. Where dolomite is present, it is

typically expressed as millimeter- to centimeter-thick laminae

or thin discontinuous beds.

Interpretation. The combination of different anhydrite mor-

phologies suggests deposition within closely related environ-

ments along an arid marine coastline. Given the large range of

nodular fabrics, it is likely that the nodular anhydrite is indica-

tive of a sabkha environment. It is important to note, however,

that not all nodular anhydrite is formed in a sabkha setting

(Warren and Kendall, 1985). The presence of isolated masses

of nodular (chicken-wire) anhydrite within other carbonates

indicates displacive growth of gypsum and anhydrite in the

upper zones of a sabkha (Warren and Kendall, 1985; Kendall,

2010). Similarly, examples of more massive nodular anhydrite

indicate that they represent an overprinted subaqueous (salina)

gypsum deposit (Kendall, 2010). The subvertical elongated

morphology of anhydrite nodules further indicates subaqueous

precipitation of gypsum in either shallow (less than 10 m)

hypersaline lagoons or brine ponds adjacent to the sabkha

(Warren and Kendall, 1985). Additional evidence of subaque-

ous sedimentation is provided by the bedded anhydrite with

interbeds of dolostone. The nature of the anhydrite, with little

evidence of subaerial exposure, implies diagenetic alteration of

primary gypsum that chemically precipitated under subaqueous

conditions (Hardie, 2003). The presence of stylolites and pres-

sure dissolution seams in some examples implies that some

nodular anhydrite formed during burial at depths of tens to sev-

eral hundred meters (Machel and Burton, 1991; Machel, 1993).

Facies 2 (F2): Microbially Laminated Mudstones

Description. Facies 2 consists of tan-colored, cryptocrystal-

line, microbially laminated carbonate mudstones (Fig. 5E).

Present within these laminated mudstones are very fine, dark-

brown to black, organic-rich laminae that vary from 5 to

50 cm in thickness. Sedimentary structures include irregular

to planar laminations and low-relief domal stromatolites

(Fig. 5F). Petrography of F2 shows the presence of silt-sized

to fine-grained sandstones within the matrix. These grains are

typically sub-rounded to sub-angular and occasionally form

FIG. 5. Core photographs of supratidal to intertidal facies in the Dunvegan gas field. All scale bars are 3 cm. A. Eroded nodular anhydrite capped by planar to

wavy black shale and laminated mudstone. The biolaminated mudstone is sharply overlain by nodular anhydrite. Well 12-11-81-4W6M, depth 1479.83m. B.

Laminated mudstone overlain by nodular anhydrite. Well 5-16-80-3W6M, depth 1478.10m. C. Subvertically elongated nodular anhydrite in an argillaceous,

brown-colored dolomitic mudstone matrix capped by massive appearing dolomudstone with dissolution seems. Well 7-18-80-3W6M, depth 1472.80m. D. Nodu-

lar mosaic anhydrite of chicken-wire juxtaposed to stylolites and interstitial mud. Well 9-25-81-5W6M, depth 1601.50m. E. Bio-laminations with wavy to con-

torted laminae, dark organic-rich partings, and stylolites. Well 7-3-81-4W6M, depth 1480.54m. F. Domal stromatolites. Well 7-3-81-4W6M, depth 1487.65m.

G.Massive patterned cryptocrystalline mudstone with alternating dark- and light-colored fabrics. Well 7-15-80-3W6M, depth 1474.52m. (See Color Plate III.)

164 G. M. BANIAK ET AL.

bedded deposits. Subaerial exposure features include desicca-

tion cracks and some fenestral (birds-eye) porosity. Skeletal

fragments and bioturbation are extremely rare in F2.

Interpretation. Fenestral porosity, formed by a combination

of gas bubbles and sediment shrinkage, are common occurrences

in tidal-flat carbonates (Choquette and Prey, 1970). Combined

with the presence of desiccation cracks, periodic subaerial expo-

sure of the substrate is envisioned in F2. The presence of

low-relief domal stromatolites also suggests deposition on high

intertidal and supratidal sites and on tidal flats with protected

shorelines (Logan, 1961; Davies, 1970; Taylor and Halley,

1974). Due to shoreline isolation, fluctuating salinities were

probable, thereby presenting additional ecological stresses to the

localized macrofauna. Because most eukaryotic organisms are

unable to withstand high salinities or exposure to extreme salin-

ity fluctuations (Franks and Stolz, 2009), the levels of bioturba-

tion and overall fossil content are expectedly limited in F2.

As a result of extreme physico-chemical conditions, flat

laminated microbial mats were able to flourish in the intertidal

flats and hypersaline lagoons of the carbonate ramp (Nicholson

et al., 1987; Stolz, 1990). The presence of organic laminae is

attributable to the sharp geochemical interface present beneath

the microbial mat (Krumbein and Cohen, 1977). Pore waters

below the interface were anoxic and thus had a higher preser-

vation potential for buried organic matter. The presence of sili-

ciclastic material within the microbial mats is likely the result

of sticky EPS produced by microorganisms (Krumbein, 1994).

As shown in the Mellum Island, southern North Sea and south-

ern coast of Tunisia (Gerdes et al., 2000), microbial mats

coated with EPS have the ability to trap and bind sediment

introduced into the tidal flats.

Facies 3 (F3): Massive Patterned Mudstone

Description. Facies 3 is recognizable in core by its distinc-

tive mottled fabric (Fig. 5G). In short, F3 is a cryptocrystalline

dolomudstone that contains no visible burrows or fossiliferous

material. Facies 3 ranges in thickness from 10 to 30 cm, with

some thicker intervals present. The mottled fabric consists of a

pattern of light- and dark-colored areas, with the concentration

of pyrite crystals seen in the darker regions. Facies 3 is com-

monly gradational with F1, F2, and F4, and is observable

throughout the Dunvegan gas field.

Interpretation. The variable size, irregular shape, and indis-

tinct contacts of the mottles suggest they are not related to bur-

rowing. Instead, the alteration of light- and dark-colored

fabrics represents a diagenetic feature wherein microcrystal-

line pyrite became concentrated in the darker regions (Dixon,

1976; Kendall, 1977b; Kirkham, 2004). Termed “patterned

carbonate” by Dixon (1976), the diagenetic origin of this

unique fabric is attributable to the destruction of calcium sul-

fate nodules by sulfate-reducing microorganisms, thereby

leading to pyrite formation (Kendall, 1977b). Within reducing

subenvironments, the formation of pyrite commonly requires

pH values greater than 8.0 and salinities above 200%

(Krumbein and Garrels, 1952). Such extreme conditions sug-

gest sedimentation of F3 within a restricted supratidal to inter-

tidal setting (Dixon, 1976), similar to those observed in the

modern Persian Gulf (e.g., Butler, 1969). Furthermore, the

cryptocrystalline dolomudstone suggests formation under

hypersaline conditions with precipitation likely induced by

sulfate-reducing bacteria (Kirkham, 2004).

Facies 4 (F4): Bioturbated Mudstone-Wackestone

Description. Bioturbated dolomudstones-dolowackestones

(60 to 100% of total reservoir volume) represent important res-

ervoir strata within the Dunvegan gas field. Common assemb-

lages include monospecific examples Chondrites,

Quebecichnus, and Planolites (Figs. 6A-B). Other ichnofossils

occasionally dispersed within F4 include moderately abundant

Thalassinoides (Fig. 6C), rare Arenicolites, rare Skolithos, and

very rare Zoophycos. Although no fossil fragments are visible

in core, petrographic sections reveal low to moderate fossil

content composed of benthic invertebrates and calcareous

algae that have been completely reworked by bioturbation.

Gradational with F2 and F3, F4 has thicknesses on the order of

decimeter to meter scale.

Interpretation. The predominance of low-diversity ichno-

logical assemblages is typical of ecological stresses, such as

high salinity and low oxygenation, commonly present within

lagoonal settings (MacEachern et al., 2007). For example, it

has been shown that the ghost shrimp Callianassa californien-

sis, which make Thalassinoides-type burrows, is capable of

tolerating fluctuating salinities (Thompson and Pritchard,

1969). Complex feeding traces (i.e., fodinichnia) are also

abundant within oxygen-poor environments (Bromley and

Ekdale, 1984; Ekdale, 1988; Ekdale and Mason, 1988). Chon-

drites, for example, is believed to be unique to anoxic condi-

tions (Bromley and Ekdale, 1984). The presence of Zoophycos

may reflect the activity of opportunistic organisms habituating

poorly oxygenated communities (Ekdale, 1988; Ekdale and

Lewis, 1991; Miller, 1991). Increased ichnodiversity present

within some intervals in F4 (e.g., Skolithos, Arenicolites) is

interpreted to be the result of events such as fresh water influx

that helped alleviate earlier stresses imparted on the ecosys-

tem. Collectively, F4 represents a restricted, low-energy

lagoon.

Facies 5 (F5): Skeletal Packstone-Wackestone

Description. Facies 5 is a bioclastic packstone-wackestone

composed of a diverse assemblage of fauna (Fig. 6D). Identifi-

able fossils include fragmented brachiopods, crinoids, bryozo-

ans, and ostracodes. Also present are stylolites, pressure

dissolution seams, and peloids. Grain sorting varies between

different intervals, ranging from poorly sorted to well sorted.

The matrix is composed primarily of carbonate mud within the

wackestone successions, and becomes increasing depleted of

mud in the more well-sorted packstone-rich intervals. Either

gradational or sharp contact with F2, F3, and F4, bed

BURROW-MEDIATED DOLOMITIZATION DEBOLT FORMATION 165

thicknesses in F5 range from 10 to 25 cm, although thicker

intervals are present.

Interpretation. The diversity of fossils suggests a normal-

marine origin (James and Kendall, 1992). The well-sorted,

fine-grained, bedded nature indicates that the skeletal material

underwent earlier wave reworking in a high-energy beach or

shoal environment. The fact that F5 is in close contact with

multiple facies (F2, F3, F4) suggests possible allochthonous

transportation of sediment (e.g., tempestites). As a result, sedi-

mentation of these skeletal packstones-wackestones may have

been due to either storm activity or minor sea level transgres-

sions that inundated the restricted intertidal to subtidal regions

of the lagoonal environment.

Facies 6 (F6): Peloidal Grainstone-Packstone

Description. Facies 6 consists of a peloidal grainstone-

packstone. Allochems include oncoids, ooids, and pisoids.

Micritic envelopes (Fig. 6E) have been identified on pisoids

and shell fragments. The peloids range in shape from ovoid

(Fig. 6F) to nonrounded. Comprising a poor to moderately

diverse assemblage of skeletal fragments, bioclasts include

ostracodes, crinoids, brachiopods, and bryozoans. The

majority of the intervals are massive bedded (Fig. 6G),

although low-angle cross-bedding is present in places. Bio-

turbation is generally preserved locally as indistinct mot-

tling. Gradational with F4 and F5, F6 has a thickness

ranging from 30 cm to 1 m.

FIG. 6. Core photographs of subtidal Debolt Formation facies in the Dunvegan gas field. All scale bars are 3 cm unless otherwise noted. A. Highly bioturbated,

monospecific assemblage of Chondrites (Ch). Well 8-13-81-5W6M, depth 1452.31m. B. Highly bioturbated dolomudstone dominated by a monospecific assem-

blage of Planolites (Pl). The interval forms the principal reservoir in the observed well, with permeabilities ranging above 40 mD. Well 9-18-81-4W6M, depth

1412.15m. C. Bioturbated assemblage of Planolites (Pl) and Thalassinoides (Th). Well 8-20-80-3W6M, depth 1497.25 m. D. Fossiliferous packstone overlain by

laminated mudstone with metasomatic anhydrite. Fossils include crinoids, brachiopods, bryozoans, and ostracodes. Well 7-15-80-3W6M, depth 1439.64m. E.

Thin section photomicrograph of peloidal grainstone facies (Pe), where the irregular shape suggest they may have been originally skeletal grainstones. Subordi-

nate amounts of ostracode (Os) and crinoids fragments exist and some of the larger grains display simple micrite envelopes (Me). Calcite cement occludes pri-

mary porosity. Well 8-13-81-5W6M, depth 1455.02m. F. Thin section photomicrograph of peloidal grainstone facies, where the rounded character of the grains

suggests fecal pellets. Calcite cement occludes primary porosity. Well 5-16-80-3W6M, depth 1457.20m. G.Massive bedded deposit of peloidal grainstone. Well

7-3-81-4W6M, depth 1492.25m. (See Color Plate IV.)

166 G. M. BANIAK ET AL.

Interpretation. The presence of irregularly shaped peloids

suggests micritization of allochems, such as skeletal fragments

or ooids (Bathurst, 1966). Peloids that are either elongated or

ovoid in shape suggests a fecal origin (Land and Moore,

1980). Preservation of the peloids indicates deposition within

a shallow, low-energy, restricted marine environment (Enos,

1983). Due to the presence of low-angle cross bedding and

fragmented fossils, F6 is interpreted to have accumulated in

either shallow peloidal shoals or low-relief banks in restricted

lagoonal environments (Qing and Nimegeers, 2008).

Petrographic Analysis

Petrographic analysis of the bioturbated fabrics (Facies 4) is

shown in Figures 7A–D. The dolomite-filled burrows, 0.5 to

2 cm in diameter, commonly contain dolomite crystal sizes

ranging from 1 to 35 microns, with an average size of 10 to 15

microns. The dolomite forms microsucrosic (planar-E) fabrics

(after Sibley and Gregg, 1987) and is generally euhedral. Dolo-

mitization within the burrows is generally thorough and a sharp

contact commonly exists between the burrow-associated dolo-

mite and calcite matrix. Furthermore, it appears that the micro-

crystalline dolomite within the burrows has been largely

preserved and not altered by burial diagenesis. Similarities in

textural composition with dolomites from the Holocene Abu

Dhabi sabkhas (McKenzie, 1981) help support this observation.

The majority of calcite within the burrows is consistent

with cement precipitated in a marine environment (e.g., blocky

calcite spar). The lime mudstone-wackestone surrounding the

burrows is dominantly fine-matrix sediment that contains

minor amounts of disarticulated and fragmented allochems.

Dolomite crystals within the matrix are similar to those

observed within the burrows, suggesting residual burrow path-

ways that were dolomitized.

Geochemistry

Isotopic analyses of calcites and dolomites are shown in

Figure 8. To represent the relationship between the calcite and

dolomite isotopes within the burrows and matrix, a straight

line (y D mx C b) was employed. From this equation, the coef-

ficient of determination, R2, can be deduced. In short, the

closer the R2 is to C 1 or -1, the better the correlation between

the isotopes being analyzed. Using R2, the calcite and dolomite

plotted as linear trends with good correlations (i.e., R2> 0.62),

thereby indicating a strong linear relationship for the calcite

and dolomite isotopes regardless of sample location (i.e.,

burrow or matrix).

FIG. 7. Thin section photomicrographs (A, C) and corresponding schematic diagrams (B, D) of dolomite-filled burrows and their surrounding calcite matrices.

Both thin section photomicrographs are from Well 9-18-81-4W6M, depth 1412.15m.

BURROW-MEDIATED DOLOMITIZATION DEBOLT FORMATION 167

Calcite isotopic ratios

Matrix calcite (n D 22) has measured d13C values ranging

from 1.6 to 5.1% (mean D 3.2%) and d18O values of ¡3.1 to

5.0% (mean D 1.4%). A similar range of values is observed

for burrow-hosted calcite (n D 13), where d13C range from 1.7

to 4.0% (mean D 3.1%) and d18O range from ¡2.7 to 2.9%(mean D 1.1%). The slope for matrix-hosted calcite (Fig. 8A)

differs only slightly from that of burrow-hosted calcite

(Fig. 8B), with values of 0.32 and 0.33 calculated for matrix-

and burrow-hosted, respectively.

Dolomite isotopic ratios

The d13C for dolomite ranges from 3.1 to 4.6% (mean D3.4%) and 3.2 to 4.3% (mean D 3.5%) for samples obtained

from the matrix (n D 12) and burrows (n D 25), respectively.

In comparison to calcite, dolomite tends to be more enriched

in d18O, with a narrower range of isotopic ratios is observed.

For matrix dolomite, d18O ranges from 0.7 to 2.9% (mean D2.1%), and d18O for burrow-hosted dolomite between 2.1 and

2.4% (mean D 2.4%). An even smaller difference in calcu-

lated slopes is present for the dolomite phases, with values of

0.82 and 0.84 for matrix-hosted (Fig. 8C) and burrow-hosted

(Fig. 8D), respectively.

DISCUSSION

Paleoenvironmental Reconstruction

The carbonate facies within the Dunvegan gas field are

comparable to the modern intertidal flats and sabkhas of the

Persian Gulf (Evans, 1966; Butler, 1969; Behairy et al.,

1991; Al-Youssef et al., 2006) and the Upper Cretaceous

intertidal facies of Hvar Island, Croatia (Diedrich et al.,

2011). The six facies described correspond to the shallow

inner and middle portions of the carbonate ramp (Fig. 9).

They extend from the peritidal setting to the shallow subtidal

zone through a low to moderate energy carbonate shoal

facies. Core analysis shows mainly gradational boundaries

within the facies, suggesting that they are genetically related

and demonstrate a low depositional gradient. For Facies 5

demonstrating an allochthonous origin, tempestites are envi-

sioned as a possible mechanism for sediment transportation

along the shallow-gradient ramp.

The lithofacies cycles in the Dunvegan gas field, commonly

capped by nodular to nodular mosaic anhydrite, are interpreted

as fourth and fifth order transgressive-regressive parasequen-

ces that represent the initial flooding and subsequent prograda-

tion of a sabkha system (Al-Aasm and Packard, 2000; Packard

et al., 2004). The presence of laminated, vertically elongated,

and massive nodular anhydrite, as described in F1, suggests

numerous other evaporitic sub-environments existed in the

backshore adjacent to the sabkha shoreline, such as hypersa-

line lagoons. The lagoonal facies are characterized by massive,

highly bioturbated packstone, wackestone, and mudstone lith-

ofacies. The thin packstone/wackestone beds that were occa-

sionally present between more massive wackestones and

mudstones may represent possible storm deposits.

Monospecific assemblages of ichnofossils, formation of

patterned carbonate, and lack of tidal structures, among other

features, supports the interpretation that the lagoon was

restricted. Isolating the lagoon from the open marine water

was peloidal shoals/low-relief banks composed of fecal mate-

rial and skeletal fragments. As the geochemical results shows,

FIG. 8. Isotopic values for calcite and dolomite from the burrows and adjacent matrix. A. Matrix-hosted calcite (n D 22) has a d13C mean of 3.2% and d18O

mean of 1.4%. B. Burrow-hosted calcite (n D 13) has a d13C mean 3.1% and d18O mean of 1.1%. C.Matrix-hosted dolomite (n D 12) has a d13C mean of 3.4%and d18O mean of 2.1%. D. Burrow-hosted dolomite (n D 13) has a d13C mean 3.5% and d18O mean of 2.4%.

168 G. M. BANIAK ET AL.

an abundance of organic carbon (burrows, biolaminated mud-

stones, EPS), coupled with periods of anoxia within the

restricted lagoon, provided favorable conditions for the devel-

opment of dolomitization.

Dolomitization Models

The d13C (3.4% matrix, 3.5% burrows) and d18O (2.1%matrix, 2.4% burrows) values for the early matrix dolomite in

this study are consistent with precipitation from hypersaline

marine fluids, as observed elsewhere by Al-Aasm (2000), Al-

Aasm and Packard (2000), and Cioppa et al. (2003) within the

Dunvegan gas field. The Mg-rich brines, likely derived from

Mississippian seawater (Bruckschen et al., 1999; Cioppa

et al., 2003), resulted in the replacement of carbonate mud by

the early matrix dolomite and precipitation of primary evapor-

ites. Correlation of isotopic values in this study with Holocene

dolomites from the Abu Dhabi sabkhas (McKenzie, 1981) sup-

port this assertion (Fig. 10). Because of the evidence support-

ing sabkha dolomitization, it is likely that other models of

dolomitization have been overlooked within the Dunvegan gas

field.In this context, it must also be noted that recrystallization,

due to burial diagenesis, does not appear to have had any sig-

nificant impact on the texture or geochemical values of the

microcrystalline dolomites preserved within the burrow fab-

rics. This is unique, given that the Debolt Formation within

the Dunvegan gas field has undergone over 300 million years

of burial history to depths of roughly 4 km and temperatures

in excess of 100�C (Al-Aasm, 2000; Al-Aasm and Packard,

2000). It has been postulated by Al-Aasm (2000), Al-Aasm

and Packard (2000), and Packard et al. (2004) that hydrody-

namic isolation, geochemically closed diagenetic systems, and

hydrocarbon charging each played a significant role in helping

preserve the microcrystalline dolomite. The exact mechanism,

however, remains up for debate.

Given the data presented above, burrow-mediated dolomiti-

zation is being asserted in this study as an additional dolo-

mite-promoting mechanism. This is because we ascribe the

compositional differences observed in the isotopic distribu-

tions to reflect the different biogeochemical processes occur-

ring within and adjacent to the burrows (Gingras et al., 2004;

Konhauser and Gingras, 2007, 2011; Rameil, 2008; Lalonde

et al., 2010; Petrash et al., 2011; Corlett and Jones, 2012).

Parameters Influencing Dolomitization

When the isotopic values for calcite and dolomite are

plotted together (Fig. 10), the linear trend observed for cal-

cite is maintained and the dolomite values fall into two gen-

eral groups either above or below the calcite linear trend.

These two fields likely represent dolomite that has precipi-

tated in different spatial environments within the carbonate

ramp. The scale of these spatial differences is currently

unclear.

The zone of relative enrichment of d13C in dolomite (i.e.,

values occurring above the calcite linear trend) is consistent

with a biogenically mediated phase. d13C enrichment has been

interpreted as being associated with methanogenesis (Roberts

et al., 2004) and with conditions of anaerobic fermentation

(Reitsema, 1980; Dimitrakopoulos and Muehlenbachs, 1987).

An overall enrichment of d18O in dolomite relative to cal-

cite suggests that the pore-water associated with dolomitiza-

tion may have been enriched in d18O due to evaporitic and

sabkha conditions or due to the dolomite undergoing a greater

fractionation factor relative to calcite. Low-temperature

experiments of modern dolomite precipitating in equilibrium

with calcite under conditions of MSR indicate that dolomite

should be enriched in d18O by approximately 2.6% (Vascon-

celos et al., 2005). Mean d18O values for dolomite in this study

are enriched by 0.8% relative to mean calcite values (1.2%).

This fractionation is closer to values measured in a lab than

FIG. 9. A schematic drawing of a low-angle carbonate ramp reflective of sedimentation during the Visean within the Dunvegan gas field. Superimposed on the

carbonate ramp is the location of the six facies identified during core logging. Facies 4 (bioturbated mudstone-wackestone) occurs primarily within the lagoonal

portions of the inner ramp and represents the primary reservoir intervals.

BURROW-MEDIATED DOLOMITIZATION DEBOLT FORMATION 169

values extrapolated to low-temperature conditions from high-

temperature experiments (e.g., Epstein et al., 1964; Northrop

and Clayton, 1966; O’Neill and Epstein, 1966). The similarity

in approximated fractionation values calculated between cal-

cite and dolomite in this study with those calculated by Vas-

concelos et al. (2005) for dolomite and calcite precipitating

from the same parent solution suggests that protodolomite pre-

cipitated in equilibrium with calcite rather than by replacement

of pre-existing calcite.

There are examples of dolomitized burrows in the rock

record, however, wherein d18O dolomite is depleted relative to

calcite (e.g., Corlett and Jones, 2012). Within their study of

burrow dolomites in the Middle Devonian Lonely Bay Forma-

tion in Northwest Territories, Canada, Corlett and Jones

(2012) showed that a requisite for dolomitization, among

others, was a high concentration of marine-dissolved organic

matter. On the other hand, the burrows within the Debolt For-

mation lack marine-derived organic matter due to a sabkha

based depositional setting.

As a result, a significant portion of the organic content

within the Debolt Formation is most likely derived in part

from the organisms and their by-products (i.e., fecal material,

burrow wall linings) occurring within a lagoonal setting.

Consequently, the distribution of d18O in dolomite relative to

calcite within burrows, at present, appears to be influenced the

source of organic matter and depositional setting. Clearly, fur-

ther work needs to be completed to better define these bound-

aries throughout the geological rock record.

Less depleted d13C values within the burrow dolomites in

the Dunvegan gas field are thought to result from dolomite

formed either: (i) in association with early sulfate reduction

(i.e., before pores become saturated with d13C) or (ii) in sedi-

ments with little organic matter, where much of the CO32¡ is

derived from seawater (Compton, 1988; Mazzullo, 2000). It

has been noted that d13C for dolomite precipitated in associa-

tion with sulfate reduction in modern settings is initially very

similar to d13C of calcite, but becomes progressively more

depleted as sulfate-reducing conditions persist (Vasconcelos

et al., 1995; Mazzullo, 2000). Studies of dolomite found

associated with MSR in a modern lagoonal environment

(e.g., Lagoa Vermelha, Brazil) suggest that dolomite is ini-

tially slightly d13C enriched relative to organic carbon, and

becomes progressively depleted by as much as 5% upon

shallow recrystallization (Vasconcelos and McKenzie,

1997). For this study, the majority of d13C values fall within

1% of seawater, although a few samples are as much as 2%

FIG. 10. Composite plot showing the average linear trend for calcite and the two distinct zones of dolomite occurring either above or below the calcite linear

trend line. Observed d18O and d13C values from the burrow and matrix dolomites in this study are comparable to protodolomities values measured by McKenzie

(1981) in the modern Abu Dhabi sabkhas and microdolomite values measured by Al-Aasm and Packard (2000) from the Dunvegan gas field.

170 G. M. BANIAK ET AL.

lighter. The similarity of d13C values measured in the present

study with those measured in modern field and laboratory

dolomite associated with MSR suggest that the zone of isoto-

pically light dolomite (i.e., values occurring below the linear

calcite trend) most likely formed under early sulfate reducing

conditions.

The high degree of similarity of burrow and matrix dolo-

mite d13C to seawater values suggests that dolomite formed

near the sediment-water interface, under conditions of early

sulfate reduction. This is envisaged as occurring potentially

within a burrow environment, where sulfate reduction can

occur adjacent to or within a burrow (Waslenchuk et al.,

1983). We therefore advocate that heterogeneities within the

substrate, such as those made by bioturbation, helped promote

dolomitization within the Dunvegan gas field.

The presence of anoxia within the lagoonal portions of the

carbonate ramp is supported the presence of monospecific

assemblages of burrows such as Chondrites and Zoophycos

(Facies 4) (Bromley and Ekdale, 1984; Ekdale and Mason,

1988; Miller, 1991). High concentrations of pyrite (Facies 3)

and organic matter preserved within the microbial mats

(Facies 2) also confirm anoxic conditions. Given these geo-

chemical conditions, it can be suggested that bottom seawater

and uppermost sediment pore waters (i.e., burrowed zone)

fluctuated from oxic to anoxic. The margins of the burrows

walls, due to high organic carbon content, are also believed to

have formed a localized reducing zone that allowed for MSR

and localized dolomite precipitation.

From a reservoir viewpoint, early dolomitization was criti-

cal in preserving the micro-intercrystalline porosity within the

burrow fabrics (Al-Aasm and Packard, 2000). Moreover, these

dolomitized burrow fabrics commonly provide localized

enhancement of permeabilities relative to the impermeable

lime mud matrix. As shown by Gingras et al. (2007b, 2012)

and Baniak et al. (2013), dolomitized bioturbated media com-

monly develop anisotropic permeability networks. Given the

lateral extensiveness of these bioturbated intervals across the

Dunvegan gas field (Fig. 4), proper identification of the dolo-

mitized burrow fabrics is crucial to drilling success and opti-

mal recovery.

CONCLUSIONS

In the Dunvegan gas field of northwestern Alberta, the Mis-

sissippian (Visean) Debolt Formation contains numerous indi-

cators of sedimentation within a carbonate ramp setting.

Among others, these include reoccurring intervals of patterned

carbonate, monospecific assemblages of ichnofossils (e.g.,

Chondrites, Planolites), nodular anhydrite, biolaminated mud-

stones, and peloidal grainstones. The reduction of both fossils

and burrows within the lagoonal settings indicates that phys-

ico-chemical stresses, such as reduced oxygen and fluctuating

salinities, were prevalent during the Visean. These

parasequence surfaces are laterally extensive and correlatable

across the Dunvegan gas field.

Isotopically, the Dunvegan gas field provides an excel-

lent example of early dolomitization within a sabkha setting.

However, evidence for burrow-mediated dolomitization was

also established when comparing calcite and dolomite iso-

tope data from the burrows and adjacent matrix. The isoto-

pic values for calcite follow a linear trend on a d13C versus

d18O plot, while dolomite plots into two distinct zones either

above or below the calcite linear trend. Dolomite that is

d13C enriched relative to calcite is interpreted to be dolo-

mite that formed in a zone of methanogenesis. The majority

of dolomite measured, however, was depleted in d13C rela-

tive to average calcite values. This amount of isotopic

depletion is similar to samples that are measured in labora-

tory and field-based studies where dolomite has been found

in association with bacterial sulfate reduction coupled to the

oxidation of organic carbon. High organic content, an essen-

tial component for MSR and subsequent near-surface dolo-

mitization, is believed to have been derived in part from the

organisms and their by-products (i.e., fecal material, burrow

wall linings).

ACKNOWLEDGMENTS

We would like to thank Ichnos editor Luis Buatois, along

with reviewers Hilary Corlett and Adrian Immenhauser, for

their helpful comments and suggestions that improved the final

version of this paper. Jenni Scott and David Morrow are

acknowledged for their editorial input and helpful suggestions

during earlier versions of this draft. We would like to thank

Mark Labbe and David Pirie for their help regarding thin sec-

tion preparation. The staff at the ERCB in Calgary, Alberta, is

also recognized for their support and help during our stay at

the core lab in summer 2010.

FUNDING

We are grateful for the financial support offered by Cono-

coPhillips, Devon Energy, and Statoil.

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